When a computer system or other data terminal equipment ("DTE") is connected to a remote DTE or computer system via a public telephone network, a modem is typically used to connect the DTE or computer system to the telephone network. The function of the modem is to convert a digital signal into an analog signal that is suitable for transmission over the telephone network, and to convert an analog signal back into a digital signal.
FIG. 1 illustrates such a prior art communication system 10. As can be seen from FIG. 1, modem 11 is connected to a DTE 12 and an external network 15 and modem 13 is connected to a DTE 14 and network 15. Modem 11 includes a transmitter circuit 11a, a receiver circuit 11b, and a hybrid circuit 11c. Similarly, modem 13 includes a transmitter circuit 13a, a receiver circuit 13b, and a hybrid circuit 13c. When DTE 11 wants to send data to DTE 14, transmitter circuit 11a of modem 11 receives the transmitting data. Transmitter circuit 11a then processes and modulates the transmitting data into the transmitting signal suitable to be transmitted via network 15. The transmitting signal is then transmitted to modem 13 via hybrid circuit 11c and via network 15. When modem 13 receives the transmitting signal from modem 11 (now the receiving signal of modem 13), hybrid circuit 13c of modem 13 sends the receiving signal to receiver circuit 13b. Receiver circuit 13b then demodulates and processes the receiving signal into digital data which are then fed to DTE 14.
When DTE 12 receives data from DTE 14, DTE 14 sends the data to transmitter circuit 13a of modem 13 which are then processed and modulated to be sent to network 15 via hybrid circuit 13c. Network 15 then sends the modulated data to receiver circuit 11b of modem 11 via hybrid circuit 11c. Receiver circuit 11b then demodulates and processes the modulated data received from modem 13. The data are then applied to DTE 12.
Each of modems 11 and 13 is a full duplex modem. This means that each of hybrid circuits 11c and 13c can transmit and receive modulated data signal simultaneously.
When receiver circuit 11b of modem 11 receives a receiving signal from network 15 via hybrid circuit 11c, the receiving signal typically includes the modulated data transmitted from modem 13 and an echo signal of the transmitting signal of modem 11. The echo signal typically includes a near end echo signal and a far end echo signal. FIG. 1 shows the signal path of the near end echo signal and the far end echo signal.
As can be seen from FIG. 1, the near end echo signal is received in receiver circuit 11b through transmitter circuit 11a and hybrid circuit 11c. The far end echo signal is, however, the reflection of the transmitting signal of transmitter circuit 11a from network 15. As can be seen from FIG. 1, the far end echo signal is the transmitting signal of modem 11 that is transmitted to network 15 via hybrid circuit 11c and then echoed back from network 15 to receiver circuit 11b via hybrid circuit 11 c.
In order to cancel the near and far end echo signals from the receiving signal in receiver circuit 11b, a near end echo replica signal generation circuit and a far end echo replica signal generation circuit are typically provided in the modem. The near end echo replica signal generation circuit simulates the characteristics of the near end echo signal path and generates the replica signal of the near end echo signal. The far end echo replica signal generation circuit simulates the far end echo signal path and generates the replica signal of the far end echo signal. The replica signals are then used to cancel the echo signals from the receiving signal in the modem. FIG. 2 shows one prior art scheme of canceling the echo signals.
In order for replica signal generation circuits 24 and 26 to track variations along the echo signal paths, the output of echo canceller 27 of FIG. 2 is applied back to circuits 24 and 26 as feedback to adjust circuits 24 and 26 such that the replica signals can also track the variations of the echo signals. One disadvantage of this arrangement, however, is that the adjustment gain of circuits 24 and 26 by the output of echo canceller 27 is typically relatively low, thus causing the tracking of circuits 24 and 26 to be relatively slow. The relatively slow tracking is due to the fact that the output signal of echo canceller 27 is mainly the modulated data signal from the remote modem and the residual echo signals only constitute a relatively very small portion of the output signal of echo canceller 27. This typically causes the prior art echo canceling arrangement not to be able to track fast phase jitter occurred along the far end echo signal path. In addition, another disadvantage of the prior art arrangement is that due to the relatively extremely small adjustment gain, double precision arithmetic is typically required to process the tracking loop, thus increasing the overall processing complexity and time.